The invention relates to a process for the reduction of the carboxyl end groups of linear saturated polyesters by means of reaction with alkylene carbonate in the melt phase in the presence of a phosphonium catalyst.
Tough yams for technical applications, such as cords for motor vehicle tires and conveyor belts, are manufactured from particularly high-molecular polyesters. Such high-molecular polyesters are produced through the post-condensation of precondensate, either in the solid phase (U.S. Pat. No. 4,064,112) or in the melt phase, in specially-designed reactor units (U.S. Pat. Nos. 3,728,083 and 5,055,273). The higher the carboxyl end group concentration of the prepolymer, however, the lower is its ability for post-condensation. A reduction of the carboxyl end groups preceding polycondensation, in order to increase the polycondensation ability, may thus be necessary in the production of particularly highmolecular polyesters.
The thermal stability of polyester drops sharply, not only with increasing temperature and residence time but also, most particularly, with increasing molecular weight, which leads to the breakdown of the molecular chains, and as a result to the formation of additional carboxyl end groups. Thus, the carboxyl end group concentration of the polyester discharged from the post-condensation reactor is usually over 20 meq/kg and, in the finished yam, over 25 meq/kg. By means of particular equipment measures, values of approximately 16 meq/kg can be achieved in the yarn (U.S. Pat. No. 4,867,936). At the present time, however, the market requires yarns with less than 15 meq/kg and, if at all possible, less than 12 meq/kg, of the carboxyl end groups.
The prior art discloses reduction of the carboxyl end groups through the reaction of the polyester in the melt with different reagents, such as low-boiling oxiranes (U.S. Pat. No. 4,442,058), polycarbonates (U.S. Pat. No. 4,171,422), or alkylene carbonates (U.S. Pat. No. 4,348,314 and JP-B 48-041 713). Oxiranes, because of their low boiling point (under 75.degree. C.), can only be homogeneously mixed into the polyester melts with difficulty and are, in addition, highly toxic. Polycarbonates can be handled well, but through the incorporation of polycarbonate units, lead however, to a disruption of the homogenous polyester structure and, as a result, to an impairment of the yarn characteristics, which can not be tolerated, particularly in HMLS yarns (high modulus, low shrinkage yarns). As catalysts, both tetra-(n-C.sub.1-6 -alkyl)-phosphonium acetates as well as -halogenides (U.S. Pat. No. 4,171,422) are suitable for this purpose. By supplying ethylene carbonate without a catalyst into the polyester-polycondensation reactor, the polycondensation reaction can be accelerated, but without, however, influencing the content of the carboxyl end groups (U.S. Pat. No. 4,289,871). The addition of a catalyst is necessary for the reduction of the carboxyl end groups. Equally well-suited for such purposes are organic phosphines and phosphonium hydroxides, halogenides or carboxylates, in which saturated alkyl groups and aryl groups are present as organic radicals, such as, for example, butyl triphenylphosphonium chloride (JP-B 48-041713). Most of these alkyl- and/or arylphosphonium compounds are strongly hygroscopic and must be stored under nitrogen. At lower concentrations (below 200 ppm), a distinct reduction of the viscosity of the polyester appears and, at higher concentrations (400-500 ppm), in comparison with polyester without the addition of alkylene carbonate/catalyst, a yellow discoloration of the polyester additionally appears during the reaction of the polyester with alkylene carbonate. Alkali halogenides and alkali thiocyanates also show a good catalytic activity (U.S. Pat. No. 4,348,314). Alkali salts, however, promote the hydrolytic breakdown of the polyester, which can not be sufficiently compensated, particularly in respect to long-term stability, even through the addition of the phosphite- or phosphonate stabilizers (U.S. Pat. Nos. 4,374,960 or 4,374,961 ).